1. Field of the Invention
The present invention relates to wireless localization. More particularly, the present invention relates to estimating a mobile terminal's position using a mobile time-of-arrival (TOA) technique.
2. Discussion of the Related Art
Many location-based services are possible because of accurate wireless localization. Methods based on received signal strength (RSS), angle-of-arrival (AOA), time-of-arrival (TOA), and time-difference-of-arrival (TDOA) techniques are the most commonly used approaches for estimating the position of a mobile terminal.
Non-line-of-sight (NLOS) conditions significantly affect the performance of mobile localization techniques that are based on TOA. If the line-of-sight (LOS) between a mobile terminal and a base station (BS) is obstructed, a delay (“NLOS bias”) is introduced into the received signal. Even a single or a few NLOS BSs may significantly degrade localization accuracy, and thus mitigation of NLOS effects is important. One way to mitigate the effects of NLOS BSs is to identify them and to exclude them from the localization operation. However, when the number of BSs available for localization is limited, it may be impossible to exclude the NLOS BSs and still meet the requirements for localization. Localization typically requires three or more BSs for a two-dimensional (2-D) localization, and a minimum of four BSs for a three-dimensional (3-D) localization.
In addition to the NLOS biases, localization inaccuracy may result from poor or imperfect estimation of the first arriving path. Typically, to estimate a TOA, a search is initiated from the strongest path and continues backwards by comparing samples against a pre-defined threshold. This search-back algorithm may lock onto an earlier or later path than the first path, and thus introduces additional biases. First-arriving-path estimation techniques are disclosed, for example, in (a) U.S. Patent Application Publication 2003/0174086, entitled “Determining a Time of Arrival of a Sent Signal,” published on Sep. 18, 2003, (b) U.S. Pat. No. 6,054,950, entitled “Ultra Wideband Precision Geolocation System,” issued on Apr. 25, 2000; (c) U.S. Patent Application Publication 2006/0104387, entitled “Method for estimating time of arrival of received signals for ultra wide band impulse radio systems” filed on Nov. 15, 2004; (d) U.S. Patent Application Publication 2003/0025631, entitled “First-arriving-pulse detection apparatus and associated methods”, filed on Jul. 26, 2001; and in U.S. Patent Application Publication 2004/0235499, entitled “Ranging and positioning system, ranging and positioning method, and radio communications apparatus”, filed on Feb. 18, 2004.
The location of a mobile terminal can also be estimated directly from the received signals. Direct estimation of the mobile terminal location is found, for example, in the article “On time-of-arrival positioning in a multipath environment”, by Y. Qi and H. Kobayashi and H. Suda, published in IEEE Trans. Vehic. Technol., vol. 55, no. 5, pp. 1516-1526, September 2006. Qi et al. teaches that the mobile terminal location can be estimated directly from the received multipath components, when a perfect knowledge of the channel parameters is available. Such knowledge of channel parameters is difficult to obtain in practice. Another direct estimation technique is presented in the article “Direct Position Determination of Narrowband Radio Frequency Transmitters,” by A. J. Weiss, published in the IEEE Signal Processing Lett., vol. 11, no. 5, pp. 513-516, May 2004. Weiss targets narrowband radio frequencies. Similarly, the article “New Direct Approaches to Robust Sound Source Localization”, by Y. Rui and D. Florencio, published in Proc. IEEE Int. Conf. on Multimedia and Expo (ICME), vol. 1, Baltimore, Md., July 2003, pp. 737-740, describes a direct approach for robust sound source localization. The article “Adapting the Ranging Algorithm to the Positioning Technique in UWB Sensor Networks”, by L. Reggiani, M. Rydstrom, E. G. Strom, and A. Svensson, published in Proc. 4th COST 289 Workshop, Goteborg, Sweden, April 2007, discloses a ranging algorithm that produces several distance estimates with associated likelihoods (rather than a single distance estimate) to improve the localization algorithm. In particular, Reggiani et al. use a projections-onto-convex-sets (POCS) algorithm for localization.
Once the TOAs of the received signals at different BSs are obtained, terminal location can be estimated using a well-known algorithm, such as a least-squares (LS) algorithm or a maximum likelihood (ML) algorithm. NLOS effects adversely affect the accuracy in the estimation of mobile terminal location. Numerous techniques have been developed that mitigate NLOS effects using information from the mobile network (and ignoring channel statistics). If all the BSs are within LOS, the residual location errors are small. However, even if only a single NLOS BS is present, the residual error may increase considerably, depending on the NLOS bias. Thus, the residual error may be used to detect the presence of NLOS BSs and to mitigate their effects. U.S. Patent Application Publication 2004/0127228, entitled “Method for Correcting NLOS Error in Wireless Positioning System”, filed on Dec. 30, 2002, discloses NLOS effects and mitigation techniques using the mobile communication network.
The article “A Non-Line-of-Sight Error Mitigation Algorithm in Location Estimation”, by P. Chen, published in Proc. IEEE Wireless Commun. Networking Conf. New Orleans, La., pp. 316-320, vol. 1, September 1999, discloses a technique for suppressing NLOS signals using the TOA information and the mobile network. The technique assumes that NLOS identification is not possible from the received multipath signal, and the number of BSs is greater than the minimum required. The article further discloses using different combinations of BSs to obtain location estimates, which are then weighted by the inverse of the corresponding residuals to obtain a final location estimate.
U.S. Patent Application Publication 2006/0125695, entitled “System and Method for Enhancing the Accuracy of a Location Estimate”, published on Jun. 15, 2006, discloses a general framework for “enhancing the accuracy of a location estimate that modifies weights in a weight matrix associated with receiver station measurements in parallel with successive refinements of the location estimate”. In one implementation, the weights are obtained from the residuals. Information from statistics on multipath-received signals is not used.
Numerous papers disclose techniques for NLOS identification using the statistical distribution of the TOA of the received signal. Such techniques, which typically assume that the mobile terminal is in motion, require a large number of measurements to accurately capture the necessary statistics for NLOS identification. For a static terminal, however, such techniques may not provide reliable identification, and multipath statistics of the received signal have to be used. NLOS identification using multipath received signals in a code division multiple access (CDMA) system is disclosed in European Patent Application Publication EP 1,469,685, entitled “a Method distinguishing line of sight (LOS) from non-line-of-sight (NLOS) in CDMA mobile communication system”, filed on Dec. 29, 2002. In this patent application, a channel is identified as a LOS channel, if the power ratio of the maximum path to the local maximum path is greater than a pre-defined threshold and, simultaneously, the arrival time difference between the first path and the maximum path is less than a given time interval.
U.S. Patent Application Publication 2005/0281363, entitled “Wireless Positioning Approach Using Time Delay Estimates of Multipath Components”, filed on Jun. 8, 2005, observes that, besides the first arriving signals, the second and later arriving signals, which are created due to NLOS propagation, also carry information regarding the position of interest. Therefore, all available multipath components may be processed, along with the first arriving ones, to improve localization accuracy.
Typically, triangulation in a localization operation is achieved in two independent steps; 1) TOAs are estimated from the multipath received signals corresponding to each terminal and base station link, and 2) based on the estimated TOAs, a least-square solution is obtained for mobile terminal's location. Note that, after estimation of the TOAs, information regarding the received signals is discarded, and only the TOA estimates are passed to the triangulation step. However, as stated in U.S. Patent Application Publication 2005/0281363, discussed above, the multipath components of the received signal also carry useful information regarding the LOS or NLOS characteristics of a terminal and base station link. When the à priori probability density function of NLOS induced path lengths is available, the probability density function (PDF) may be used to obtain a maximum à posteriori (MAP) estimate of mobile terminal location.
Direct location estimation techniques have either large computational complexities or high a-priori knowledge requirements. For example, the method disclosed by Qi et al., discussed above, requires knowledge of a channel impulse response (CIR) which is difficult to obtain in practice. A technique that does not require knowledge of the CIR is therefore desired.